1. Field of the Invention
The present invention relates generally to pseudo-random noise sequence generators, and in particular to pseudo-random noise sequence generators that generate a spectrally shaped noise sequence.
2. Background
Pseudo-random noise (PN) sequences are used in many applications. Among them is a delta-sigma modulator. A delta-sigma modulator utilizes a combination of over-sampling and spectral shaping of quantization noise to achieve a high-resolution data conversion despite using a coarse quantization. The principle of delta-sigma modulation is well known and thus not described in detail here. It is also well known that a delta-sigma modulator is prone to a problem known as “limit cycle oscillations,” which is observed when the output exhibits an unwanted periodic pattern within a time span. “Limit cycle oscillations” are highly undesirable. In audio applications limit cycle oscillations may be audible. Limit cycle oscillations has been an active area of research to alleviate this problem. The most common approach to avoid “limit cycle oscillations” is to use “dithering”, which injects a PN sequence into the modulator and thus breaks up an otherwise periodic output pattern. This effectively suppresses “limit cycle oscillations” but unfortunately also increases the noise floor of the modulator. The noise floor is the sum of all noise sources and unwanted signals. Other sources of noise include thermal noise sources within a system. Increasing the noise floor reduces the minimum signal level that can be detected in a system. There is unavoidably a trade-off between “spurious tones” (due to limit cycle oscillations) and noise floor in prior art delta-sigma modulators.
Prior art delta-sigma modulators use a PN sequence generator constructed by a linear feedback shift register (LFSR). A LFSR consisting of N-bit shift registers is able to generate a repetitive sequence of 2N−1 binary numbers. The power spectral density (PSD) of a PN sequence is white, i.e., all spectral components are of equal power. Delta-sigma modulators, on the other hand, rely on spectral shaping to suppress the noises in the signal band of interest thus achieving a high-resolution data. The injection of a white PN sequence into the delta-sigma modulator adds a white noise sequence to the modulator and thus does not fully comply with the underlying principle of a delta-sigma modulator. In some prior arts, a PN sequence is injected immediately before the internal quantizer of the modulator, and the injected PN noise is thus also spectrally shaped along with the quantization error. This may help to suppress the in-band components of the injected PN sequence. However, the in-band noise floor still increases, compared to the case wherein no PN sequence is injected.
What is needed is a spectrally shaped PN sequence generator that has very low in-band spectral components and the effect of dithering on the in-band noise floor is thus negligible.